It is known that ammonium salts of certain kinds, such as imidazolium salts and pyridinium salts, become liquid molten salts at 100° C. or lower, particularly around room temperature, and exhibit high ionic conductivity at relatively low temperatures of 200° C. or lower even in the absence of water or an organic solvent. In view of the unique non-volatility of these molten salts, application as an electrolyte of batteries, etc. has been studied. Being liquid, however, they are not easy to handle. In order to facilitate handling of the molten salts, several proposals have been made on a polymer electrolyte comprising a molten salt immobilized with a polymer.
For example, JP-A-8-245828 discloses a composition comprising an aliphatic quaternary ammonium salt of an organic carboxylic acid and a polymer, such as polyvinyl chloride, polyacrylonitrile or an aliphatic polyether. JP-A-7-118480 discloses a combination of a room-temperature molten salt and a polymer of a vinyl monomer having an alkyl quaternary ammonium salt structure. JP-A-10-83821, JP-A-2000-3620, and JP-A-2000-11753 propose an aliphatic molten salt type polymer synthesized from an imidazolium compound and an acid or an acid monomer. A. Noda, et al., Electrochim Acta, vol. 45, 1265 (2000) and JP-A-11-86632 report a composition of a vinyl polymer and a molten salt. Because all these compositions use a polymer the main chain of which is composed mainly of an aliphatic hydrocarbon group, they are inferior in durability characteristics including resistance to oxidation.
JP-A-11-306858 discloses a composition of a vinylidene fluoride polymer and an imidazolium salt, and J. Electrochem. Soc., vol. 147, 34 (2000), Electrochimica Acta, vol. 46, 1703 (2001), and JP-A-11-86632 teach compositions comprising a perfluoropolymer having an acid group and a molten salt. A composition containing a fluoropolymer is expected to have improved durability but is disadvantageous from the point of view of cost and environmental burdens involved in the production of fluoropolymers. It has therefore been demanded to develop an inexpensive and durable molten salt composition containing a hydrocarbon polymer.
JP-A-10-265673 proposes a polymer composite having an ionic liquid solidified with a non-fluoropolymer. However, an electrolyte composition having a molten salt combined with an aromatic polyether ketone excellent in durability, particularly resistance to oxidative deterioration, is not mentioned in the publication nor known from any other literature. Neither does the publication suggest an electrolyte composition containing a molten salt and a polystyrene-based polymer, which is one of general-purpose resins and inexpensive.
While an aromatic polyether sulfone is inexpensive and excellent in durability such as resistance to oxidative deterioration, an electrolyte composition containing the same and a molten salt is unknown.